This invention relates to a combustion chamber for supplying heated gases to a device in which a material is dried and/or heated, and more specifically, to a combustion chamber that accommodates a split-stream of recycled gases.
Drying systems are important features in the manufacture and processing of many different materials. For example, drying systems are often used in drying wood chips during the manufacturing of particle board. Further, drying systems are of particular importance during the processing of ethanol. More particularly, after ethanol has been removed from grain during the fermentation process, it is then desirable to dry the grain to allow storage and resale of the grain for animal feed or other uses.
Typical drying systems include a combustion chamber into which natural gas and air are supplied and combusted. The heated combustion gases in the combustion chamber are then induced by a draft fan into a rotating cylindrical dryer. The material to be dried is introduced into the dryer and exposed to the current of heated gases. The dried material is then separated from the heated gas current in a cyclone separator. The remaining heated gases are then typically vented to the environment. An example of a typical drying system of the prior art is disclosed in U.S. Pat. No. 3,861,055, which is incorporated herein by reference.
A major problem with these prior art systems involves the venting of the combustion gases to the atmosphere. More particularly, these combustion gases contain various pollutants. For example, the gases oftentimes contain volatile organic compounds (VOC's), carbon dioxide (CO.sub.2), particulate and nitric oxide (NO). In addition to pollutants that result from the combustion process in the combustion chamber, pollutants can also result from the drying of the material itself. For instance, in the drying of wood chips or other organic material, particulate is often contained in the combustion gases as they are vented to the atmosphere. Because governmental standards set the level of pollutants that can be vented to the atmosphere, it is often necessary to add additional pollution control devices to the drying systems to reduce the pollutant levels in the gas stream prior to venting. These devices often are add-on oxidizers which oxidize the VOC's and particulate present in the gas stream to reduce such pollutants to an acceptable level. These pollution control devices are typically expensive to install and operate.
Some prior art drying systems have attempted to address the above-discussed problem by recycling the combustion gases. More specifically, in one type of drying system, all of the combustion gases exiting the dryer are recycled back into a combustion chamber for oxidation. Gases are also taken out of the drying system at the combustion chamber and vented to the atmosphere. Recycled gases flowing into the combustion chamber and those flowing out of the combustion chamber are run through a heat exchanger wherein the heat from the gases flowing out of the combustion chamber and to the atmosphere is transferred to the recycled gases flowing into the combustion chamber. This type of drying system suffers from various disadvantages. First, because the entire quantity of combustion gases is recycled to the combustion chamber for oxidation, this drying system operates within very narrow operating parameters. More specifically, the prior art system only operates in an optimal manner at a particular capacity of the drying system. If the capacity of the drying system varies from the particular level, the oxidation temperature of the recycled gases and the inlet temperatures of the gases to the dryer could vary substantially. Because these factors could vary over large ranges, differing levels of pollutants were vented to the atmosphere depending on the capacity at which the prior art system was run. Further, again depending on the capacity, the dryer inlet temperature could vary substantially, thus resulting in inconsistent or incomplete drying of the material.
Further, these attempts to recycle gases exiting the dryer back into the combustion chamber suffer from other disadvantages. Because the recycled gases are often introduced into the combustion chamber in a haphazard and uncontrolled fashion, the gases may interfere with the operation of and efficiency of the burner flame. Furthermore, the uncontrolled introduction of recycled gases may result in incomplete and/or inconsistent oxidation of pollutants found in the recycled gases. Likewise, the removal of gases from the combustion chamber for venting to the environment has not been executed in a controlled fashion in the prior art systems. More specifically, the temperature of the gases removed from the combustion chamber was not controlled. This resulted in less than optimal efficiency of the system and is detrimental to the heat exchangers through which the vented gases will flow.
Thus, a novel combustion chamber construction is needed to overcome the drawbacks and shortcomings of prior combustion chambers. Further, a combustion chamber is needed that can oxidize pollutants within the system so that external pollution control devices are not needed. Still further, a combustion chamber is needed that will accommodate a separated recycled gas stream so that only a desired portion of the recycled gas stream is oxidized and vented to the environment. Further yet, a combustion chamber is needed that provides an avenue for removal of recycled gases after oxidation. Still further, a combustion chamber is needed that can be used to control the temperature of the gases removed by removing the gases from a desired location within the chamber.